DataTableReader.GetChars(Int32, Int64, Char[], Int32, Int32) Method
Definition
Important
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Returns the value of the specified column as a character array.
public:
override long GetChars(int ordinal, long dataIndex, cli::array <char> ^ buffer, int bufferIndex, int length);public override long GetChars (int ordinal, long dataIndex, char[]? buffer, int bufferIndex, int length);public override long GetChars (int ordinal, long dataIndex, char[] buffer, int bufferIndex, int length);override this.GetChars : int * int64 * char[] * int * int -> int64Public Overrides Function GetChars (ordinal As Integer, dataIndex As Long, buffer As Char(), bufferIndex As Integer, length As Integer) As LongParameters
- ordinal
- Int32
The zero-based column ordinal.
- dataIndex
- Int64
The index within the field from which to start the read operation.
- buffer
- Char[]
The buffer into which to read the stream of chars.
- bufferIndex
- Int32
The index within the buffer at which to start placing the data.
- length
- Int32
The maximum length to copy into the buffer.
Returns
The actual number of characters read.
Exceptions
The index passed was outside the range of 0 to FieldCount - 1.
An attempt was made to retrieve data from a deleted row.
An attempt was made to read or access a column in a closed DataTableReader.
The specified column does not contain a character array.
Examples
The following example demonstrates the GetChars method. The TestGetChars method expects to be passed a DataTableReader filled with two columns of data: a file name in the first column, and an array of characters in the second. In addition, TestGetChars lets you specify the buffer size to be used as it reads the data from the character array in the DataTableReader. TestGetChars creates a file corresponding to each row of data in the DataTableReader, using the supplied data in the first column of the DataTableReader as the file name.
This procedure demonstrates the use of the GetChars method reading data that was stored in the DataTable as a character array. Any other type of data causes the GetChars method to throw an InvalidCastException.
using System;
using System.Data;
using System.IO;
class Class1
{
static void Main()
{
DataTable table = new DataTable();
table.Columns.Add("FileName", typeof(string));
table.Columns.Add("Data", typeof(char[]));
table.Rows.Add(new object[] { "File1.txt", "0123456789ABCDEF".ToCharArray() });
table.Rows.Add(new object[] { "File2.txt", "0123456789ABCDEF".ToCharArray() });
DataTableReader reader = new DataTableReader(table);
TestGetChars(reader, 7);
}
private static void TestGetChars(DataTableReader reader, int bufferSize)
{
// The filename is in column 0, and the contents are in column 1.
const int FILENAME_COLUMN = 0;
const int DATA_COLUMN = 1;
char[] buffer;
long offset;
int charsRead = 0;
string fileName;
int currentBufferSize = 0;
while (reader.Read())
{
// Reinitialize the buffer size and the buffer itself.
currentBufferSize = bufferSize;
buffer = new char[bufferSize];
// For each row, write the data to the specified file.
// First, verify that the FileName column isn't null.
if (!reader.IsDBNull(FILENAME_COLUMN))
{
// Get the file name, and create a file with
// the supplied name.
fileName = reader.GetString(FILENAME_COLUMN);
// Start at the beginning.
offset = 0;
using (StreamWriter outputStream =
new StreamWriter(fileName, false))
{
try
{
// Loop through all the characters in the input field,
// incrementing the offset for the next time. If this
// pass through the loop reads characters, write them to
// the output stream.
do
{
charsRead = (int)reader.GetChars(DATA_COLUMN, offset,
buffer, 0, bufferSize);
if (charsRead > 0)
{
outputStream.Write(buffer, 0, charsRead);
offset += charsRead;
}
} while (charsRead > 0);
}
catch (Exception ex)
{
Console.WriteLine(fileName + ": " + ex.Message);
}
}
}
}
Console.WriteLine("Press Enter key to finish.");
Console.ReadLine();
}
}
Imports System.Data
Imports System.IO
Module Module1
Private Sub TestGetChars( _
ByVal reader As DataTableReader, ByVal bufferSize As Integer)
' The filename is in column 0, and the contents are in column 1.
Const FILENAME_COLUMN As Integer = 0
Const DATA_COLUMN As Integer = 1
Dim buffer() As Char
Dim offset As Integer
Dim charsRead As Integer
Dim fileName As String
Dim currentBufferSize As Integer
While reader.Read
' Reinitialize the buffer size and the buffer itself.
currentBufferSize = bufferSize
ReDim buffer(bufferSize - 1)
' For each row, write the data to the specified file.
' First, verify that the FileName column isn't null.
If Not reader.IsDBNull(FILENAME_COLUMN) Then
' Get the file name, and create a file with
' the supplied name.
fileName = reader.GetString(FILENAME_COLUMN)
' Start at the beginning.
offset = 0
Using outputStream As New StreamWriter(fileName, False)
Try
' Loop through all the characters in the input field,
' incrementing the offset for the next time. If this
' pass through the loop reads characters, write them to
' the output stream.
Do
charsRead = Cint(reader.GetChars(DATA_COLUMN, offset, _
buffer, 0, bufferSize))
If charsRead > 0 Then
outputStream.Write(buffer, 0, charsRead)
offset += charsRead
End If
Loop While charsRead > 0
Catch ex As Exception
Console.WriteLine(fileName & ": " & ex.Message)
End Try
End Using
End If
End While
Console.WriteLine("Press Enter key to finish.")
Console.ReadLine()
End Sub
Sub Main()
Dim table As New DataTable
table.Columns.Add("FileName", GetType(System.String))
table.Columns.Add("Data", GetType(System.Char()))
table.Rows.Add("File1.txt", "0123456789ABCDEF".ToCharArray)
table.Rows.Add("File2.txt", "0123456789ABCDEF".ToCharArray)
Dim reader As New DataTableReader(table)
TestGetChars(reader, 7)
End Sub
End Module
Remarks
GetChars returns the number of available characters in the field. Most of the time this is the exact length of the field. However, the number returned may be less than the true length of the field if GetChars has already been used to obtain characters from the field.
The actual number of characters read can be less than the requested length, if the end of the field is reached. If you pass a buffer that is null (Nothing in Visual Basic), GetChars returns the length of the entire field in characters, not the remaining size based on the buffer offset parameter.
No conversions are performed; therefore the data to be retrieved must already be a character array or coercible to a character array.
